1. Field of the Invention
The present invention relates to brushless excitation system for synchronous dynamoelectric machines, and more particularly to means for detecting faults in the rotary electric circuits of brushless exciters.
2. Description of the Prior Art
Brushless excitation systems are now widely used for supplying direct current field excitation to synchronous dynamoelectric machines such as large alternating current generators. Such brushless excitation systems include an alternating current exciter having a stationary field structure and a rotating armature member. A rotating rectifier assembly is carried on a common shaft with the exciter armature and is connected thereto to provide a direct current output. The output of the rectifier is supplied to the field winding of the main generator which also rotates with the exciter armature and rectifier. In this way, an excitation system is provided which requires no commutator or slip rings and no sliding contacts. This arrangement is particularly useful in certain operating environments, such as power plants, refineries, and chemical process plants, where it is desirable to eliminate the brushes, commutators, or other sliding contacts of conventional machines where associated maintenance problems and sparking may be unacceptable.
A problem of great concern in brushless excitation systems is the detection of rotary circuit malfunctions such as shorted diodes or open fuses in the rotating rectifier assembly, line-to-line short circuits in the rotating armature winding, and phase-to-phase short circuits in the armature winding. Conventional fault detection apparatus which employs slip rings for transmitting the fault information cannot be used in operating environments where it is desirable to avoid the use of sparking surfaces as discussed above. Thus, it is desirable to provide some alternate means which does not employ the use of slip rings or other sparking surfaces to provide an indication of circuit disturbances within the rotary electric circuits of the brushless exciter. A line-to-line or phase-to-phase short circuit within the armature winding of the exciter, if undetected, may result in severe and costly damage to the synchronous machine of which it is a part and consequential damage to the equipment to which it delivers power. The effect is particularly serious in the case of a large synchronous generator which supplies critical power to users who may be severely damaged by a sudden power fluctuation or reduction. Therefore, early detection of a circuit disturbance within the rotary armature or rotating rectifier circuit of the brushless exciter is important since such a machine may continue to operate in apparently normal fashion even after one or more line-to-line shorts occur or one or more fuses become opened. However, a point may be reached whereupon subsequent occurrence of further circuit disturbances within the rotating armature or rotating rectifier circuits will have a cumulative effect which may result in the destruction of the exciter. It is important, therefore, for those maintaining and using the synchronous machine to become aware of such disturbances upon their occurrence so that corrective action can be initiated either immediately or at regular maintenance intervals before catastrophic failure occurs.
In conventional dynamoelectric machines, indications of malfunctions within the rotary armature winding and rotating rectifier assembly have been practical only on those machines having brushes and slip rings since these devices provide the means for communicating the malfunction information to points external of the rotor. Brushless exciters have no slip rings and, therefore, have no mechanical link by which armature winding and rectifier malfunction information can be provided. Transmission of failure data by radio telemetry is feasible, but is relatively expensive.
Presently, a strobe light arrangement has been utilized satisfactorily for detecting a failure of the fuse members within the rotating rectifier assembly. However, in this arrangement, the fuse must be located in a position for physical inspection of a fuse indicator which is actuated when the fuse is opened. As a result, the fuses are mounted on the end surface of rectifier wheels. As the synchronous dynamoelectric machines become larger, the number of fuses are increased or their size is increased, with the result that the diode wheel diameter increases and becomes prohibitively large.